Bipolar transistors are devices with two p-n junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, the two p-n junctions, i.e., the emitter-base and collector-base junctions, are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar-transistor action.”
If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because the mobility of minority carriers, i.e., electrons, in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher-frequency operation and higher-speed performances can be obtained with npn transistor devices. Therefore, npn transistors comprise the majority of bipolar transistors used to build integrated circuits.
As the vertical dimensions of the bipolar transistor are scaled more and more, serious device operational limitations have been encountered. One actively studied approach to overcome these limitations is to build transistors with emitter materials whose band gaps are larger than the band gaps of the material used in the base. Such structures are called heterojunction transistors.
Heterostructures comprising heterojunctions can be used for both majority carrier and minority carrier devices. Among majority carrier devices, heterojunction bipolar transistors (HBTs) in which the emitter is formed of silicon (Si) and the base of a silicon-germanium (SiGe) alloy have recently been developed. The SiGe alloy (often expressed simply as silicon-germanium) is narrower in band gap than silicon.
SiGe HBT technology has come of age as an important semiconductor technology for both wired and wireless telecommunication applications because of its superior analog and RF performance, together with its complementary metal oxide semiconductor (CMOS) integration capability. By employing bandgap engineering, SiGe HBTs outperform Si BJTs in nearly every important performance metric and, in several areas, provide improved performance over III–V compound semiconductor HBTs.
The incorporation of carbon, C into the SiGe heterojunction bipolar device's base region by using an epitaxy process has been carried out in the prior art to prevent the out-diffusion of boron into the adjacent emitter, collector or both the emitter and collector. The foregoing is disclosed, for example, in H. J. Osten, et al., “Carbon Doped SiGe Heterojunction Bipolar Transistor for High Frequency Applications”, IEEE/BCTM, 1999, p. 169.
Both boron, B and phosphorus, P diffusion in silicon occurs via an interstitial mechanism and the diffusion is proportional to the concentration of silicon self-interstitials formed by dopant implantation, oxidation and other like processes. Arsenic enhanced diffusion is caused by vacancies. Diffusion of carbon out of a carbon-rich region causes an under-saturation of silicon self-interstitials by a mechanism known as “kick-out”. As a result, the interstitial assisted dopant diffusion in these regions will be suppressed.
U.S. Pat. No. 6,534,371 B2 to Coolbaugh, et al., entitled “C Implant for Improved SiGe Bipolar Yield” provides a method in which C is implanted into various regions or parts of the SiGe bipolar device to control or prevent bipolar shorts between the emitter, base and the collector.
Despite the prior art mentioned above, there is a continued need to boost the performance of bipolar devices, especially SiGe HBTs. In particular, there is a need for providing a new and improved method for improving the performance of bipolar devices.